A circuit for producing a constant voltage is generally used to supply a predetermined voltage, which is different from an externally input voltage, to a semiconductor device. One type of a conventional circuit for producing a constant voltage comprises first and second P type MOS field effect transistors (each defined "P-MOSFET" hereinafter) connected in series. In the circuit, gate and drain of the first P-MOSFET are connected to source and substrate potential of the second P-MOSFET, source and substrate potential of the first P-MOSFET are connected to a first voltage input terminal, and gate and drain of the second P-MOSFET are connected to a second voltage input terminal, wherein a connecting point between the gate and the drain of the first P-MOSFET and the source and the substrate potential of the second P-MOSFET is connected to a constant voltage output terminal.
In operation, first and second voltages V.sub.1 and V.sub.2 (V.sub.1 &gt;V.sub.2) are applied to the first and second voltage input terminals, respectively. A current of the first P-MOSFET is decreased to increase an output voltage at the constant voltage output terminal, and is "zero" when the output voltage ranges a value of V.sub.1 -.vertline.V.sub.T1 .vertline. to the voltage V.sub.1, where V.sub.T1 is a threshold voltage of the first P-MOSFET. On the other hand, a current of the second P-MOSFET is "zero" when the the output voltage ranges the voltage V.sub.2 to a value of V.sub.2 +.vertline.V.sub.T2 .vertline., where V.sub.T2 is a threshold voltage of the second P-MOSFET, and is increased to increase the output voltage. When the currents of the first and second P-MOSFETs are equal to each other, a predetermined output voltage is obtained at the constant voltage output terminal in a stabilized state.
A stabilized output voltage V.sub.s is defined in the equation (1). ##EQU1## where
g.sub.m1 is a mutual transfer conductance of the first P-MOSFET, and
g.sub.m2 is a mutual transfer conductance of the second P-MOSFET.
According to the conventional circuit for producing a constant voltage, however, there is a disadvantage that a range of an output voltage is narrow, as understood from reasons to be described later.
Further, there is a disadvantage that the output voltage V.sub.s fluctuates in accordance with the threshold voltages V.sub.T1 and V.sub.T2 changed dependent on the conditions of the fabricating process of MOSFETs, as understood from the equation (1).